The mercapto-substituted pyridine compound of the present invention is characterized by having an aminocarbonyl group on the pyridine ring, said aminocarbonyl group being substituted by two alkyl groups. Certain substituted pyridine compounds are disclosed in the respective specifications of U.S. Pat. No. 3,759,932; U.S. Pat. No. 4 435,206: U.S. Pat. No. 4,518,776 and U.S. Pat. No. 4,521,597. However, the above specifications fail to disclose the aforementioned mercapto-substituted pyridine compound on the pyridine ring. Also U.S. Pat. No. 4,548,942 discloses 2-mercaptonicotinamides represented by formula (4) in column 7. These compounds are useful for preparing derivatives of (5,4b)-isothiazolo pyridine-3-one as an anti-acne agent. The patented compounds do not chemically overlap with those of the present invention, the aminocarbonyl group being substituted by only one alkyl group, and the disclosed utility is quite different from the present invention.
European Patent Application Laid-Open Nos. 237,292 and 232,067 disclose a process for preparing APCA. However, the process disclosed therein is not always satisfactory industrially because of low yield, increased reaction steps, necessity of isolation in respective reaction steps, or of using reactants which are difficult to handle and expensive. The present inventors made various studies in order to find an industrially advantageous process for preparing APCA and, as a result, found out that a process by way of ACPS or salts thereof according to the present invention is capable of providing the intended effects to complete the present invention.